(a) Field of the Invention
The present invention relates to a process for refining 1,3-dimethyl-2-imidazolidinone (hereinafter referred to simply as "DMI") and a process for preparing an aromatic polyamide from an aromatic diisocyanate and an aromatic dicarboxylic acid by using DMI, as a reaction solvent, which has been refined by the above-mentioned process.
(b) Description of the Prior Art
DMI which is a five-membered compound containing nitrogen is utilized as a solvent for various reactions, because DMI is a high-boiling non-protonic polar compound having less toxicity and excellent properties.
Manufacturing processes of DMI are disclosed in Japanese Patent Laid-open Publication Nos. 175170/1982 and 172862/1986 and elsewhere, and high-purity DMI is obtained finally by distillation and is on the market.
However, even commercially available high-purity DMI contains not only water but also protonic compounds which are by-products such as biuret, urea, acetamide and N-methylformamide (hereinafter referred to simply as "NMF") as impurities in an amount of 50 to 1,000 ppm. The contents of these impurities depend upon manufacturing conditions of DMI and subsequent distillation conditions.
When DMI is used together with a raw material such as an isocyanate which easily reacts with the protonic compound, a satisfactory effect cannot be obtained at times.
Particularly, in the case that commercially available DMI is used as a polymerization reaction solvent for, e.g., an isocyanate, the protonic impurities present in DMI usually react with an isocyanate group, and so they work as short-stoppers. In consequence, a polymer having a suitable polymerization degree cannot be stably obtained.
Japanese Patent Laid-open Publication Nos. 90517/1986 and 204218/1986 disclose that substantially colorless polyamides having relative high molecular weight can be prepared by using DMI as a solvent. However, even according to these processes, the polyamides having a sufficiently high polymerization degree cannot be obtained, since the protonic impurities present in DMI react with the isocyanate group.
Thus, in such a case, it is necessary to remove the protonic impurities from DMI as much as possible.
As a technique for refining DMI, there is a distillation separation process, but since the above-mentioned impurities have about the same vapor pressure as DMI has, it is difficult to distill off substantially all of these impurities.
In fact, when commercially available DMI was further refined by distillation, it was hard to perfectly separate and remove the trace amounts of the above-mentioned impurities therefrom. For the perfect refining, it would be necessary to perform rectification in a great reflux ratio by the use of a multistage distillation column.
Moreover, when the distillation refining was carried out after the addition of an alkaline metal hydroxide such as sodium hydroxide or potassium hydroxide, or an acid such as sulfuric acid, phosphoric acid or oxalic acid, the effect of such an additive was not so perceptible.
In addition, the refining process of using a prevalent adsorbent such as active carbon, zeolite (Na.sub.2 Al.sub.2 Si.sub.3 O.sub.10 .multidot.xH.sub.2 O), silica gel (SiO.sub.2 .multidot.nH.sub.2 O) or active china clay was tried, but it was impossible to selectively adsorb the above-mentioned impurities thereby, and the effect of such an adsorbent was not observed.
Accordingly, in order to sufficiently remove the protonic impurities from DMI, time and cost are required in large quantities.